FIELD OF THE INVENTION
The invention lies in the electrical and electronics fields. Specifically, the invention pertains to a switched-mode power supply with a switching element through which a rectified AC voltage can be applied to a primary winding of a transformer, at the secondary winding of which a rectified output voltage to be controlled can be tapped. The power supply further has a control device through which the switching element can be clock-pulse controlled in dependence on a control signal. The transformer contains an E-shaped core with a middle limb surrounded by the primary and the secondary winding.
Switched-mode power supplies are known devices used to generate one or more different load-independent DC output voltages from a single rectified AC mains voltage. For this purpose the primary side rectified AC voltage is applied in a clocked manner over a switching element to the primary winding of a transformer. The output voltage can be tapped at the secondary winding after rectification and smoothing.
The output voltage is regulated through the clock-pulse control or duty cycle of the switching element. In German published patent application DE 44 07 709 this is done by tapping a control signal from an additional primary winding, whereby regulation of the clock frequency and thus of the secondary side output voltage are dependent on the control signal. The inadequate coupling between the primary side control winding and the secondary side load winding is problematical. The result is that the output voltage cannot be held sufficiently constant at load changes.
A so-called current pump for sinusoidal current input is provided in the circuit disclosed in the afore-mentioned document. Influences of the mains frequency are amplified through the current pump. A mains system hum is therefore superimposed on the output voltage and the mains system hum cannot be completely compensated because the control voltage is tapped on the primary side.
As a remedy, the control voltage can be tapped on the secondary side, as shown for example in the German utility model (Gebrauchsmuster) G 94 10 995. An optical coupler is necessary for the transmission of the control signal to the control device on the primary side. An optical coupler module is relatively costly, however, and that solution is therefore uneconomical.